A digital mixer having a touch and turn function has been conventionally known (refer to NPL1). The touch and turn function is a function to display a plurality of virtual knobs to which various kinds of parameters have been already assigned, on a display screen of a display device adapted to detect touch operation by a user, and in response to touch to one of the virtual knobs by a user's finger, assign the parameter of the touched virtual knob to a touch and turn real knob provided on an operation panel, and modify a value of the assigned parameter according to operation of the touch and turn real knob.
Further, in an apparatus provided with a function of assigning a parameter to a real knob (similar to the touch and turn function), a function of locking the assignment has also been known (refer to NPL2). This apparatus is structured such that a lock button is provided near a real knob in an audio interface apparatus in which a parameter of a virtual knob, among virtual knobs on a screen, pointed by a mouse pointer is assigned to the real knob and the value of the assigned parameter is incremented and decremented according to operation of the real knob. When a user turns on the lock button after placing the mouse pointer on one of the virtual knobs, the assignment of the parameter of the virtual knob to the real knob at the time of the turning on is locked (fixed). While the assignment is locked, the assignment of the parameter to the real knob is not changed even when the user moves the mouse pointer to another virtual knob. When the user turns off the lock button, the assignment is unlocked, and the original operation (that is, the operation in which a parameter of a virtual knob pointed by the mouse pointer is assigned to the real knob) is restored.